1. Field of the Invention
The invention relates to a method for producing steel rolling bearing rings.
2. Discussion of the Prior Art
The material with the DIN designation 100 Cr 6 and corresponding grades of steel conforming to other standards and sets of regulations from which rolling bearing rings are predominantly produced in Europe as a starting product for rolling bearings are classified as hypereutectic steels because of the high carbon content. For producing a hot-fabricated tube as a starting material for the production of individual rolling bearing rings, the following methods are used.
Starting with pig iron, an ingot is cast by means of the LD steel plant and a ladle furnace as well as a ladle A degassing facility or, alternatively, starting from an E steel plant, a ladle furnace and a ladle degassing facility and, in special cases, by means of a re-melting steel plant, and is rolled on a cogging train into a tube billet. This tube billet is preferably formed into a hot-fabricated tube by means of the Assel method (see in this respect Stahlrohr-Handbuch [steel tube manual], 10th edition, Vulkan-Verlag, Essen 1986, pages 141-143). The Assel train usually has a rotary hearth kiln as the heating installation, which is followed by a piercing apparatus, in the form of a cross-rolling mill, for producing a hollow body. This hollow body is fed to an Assel mill, comprising three inclined rolls which are arranged in uniform distribution around the circumference and are provided with a shoulder calibration. After drawing out the rod serving as an internal mold, the intermediate tube is subsequently heated and the hot-fabricated tube is produced by means of a multistand reducing mill and a downstream sizing mill.
A disadvantage of this method is that the tube billet used must be of similar dimensions to the hot-fabricated tube and a large number of rolled or forged tube billets are required to cover the supply range.
Although the Assel train is the preferred installation for producing rolling bearing tubes, also in use are other tube producing installations, such as push bench installations or continuous tube installations, always using preformed and homogenized feedstock.
Instead of an ingot, it is also known to produce a bloomxe2x80x94predominantly in rectangular formatxe2x80x94and to form it into a tube billet by means of a rolling or forging process. Alternatively, instead of a rectangular format, a round bloom is produced, this bloom also being rolled or forged after cutting off (see La Revue de Mxc3xa9tallurgie CIT [CIT review of metallurgy], April 1989, pages 344-350). According to the prior art, the degree of forming is chosen such that a degree of forging or degree of rolling of xcex=5 is achieved. The rolling or forging process mentioned is always preceded by a homogenization process, in order to remove or reduce to a great extent the segregations and coarse carbide precipitations caused by the casting process. All the methods mentioned of producing the raw material are costly, since large capital-intensive installations are required for the forming and the material has to be moved several times. Since the stretching processes make it necessary for the bars to be repeatedly divided, a corresponding amount of scrap material also occurs. Each additional working and transporting step means that there is a risk of production being affected by further or compounded errors, the elimination of which increases costs.
In German Patent 37 38 858 it is pointed out that the hot-rolled preliminary tubes are subjected to a spheroidizing long-time annealing operation before their inside and outside diameters are reduced to the desired final dimensions by means of cold pilger rolling or cold drawing, in order to transform the cementite embedded in the structure in lamellar form into globular cementite. However, this long-time annealing causes comparatively thick layers of scale to form on the outer and inner surfaces of the preliminary tubes and the surface zones are severely decarburized. The preliminary tubes are therefore peeled with respect to their outside diameter, removing the outer layers of scale.
Following this peeling, the preliminary tubes are conditioned, i.e. they are pickled, bonderized or greased. After cold pilger rolling or cold drawing, the tubes, known at this stage as rods, are placed into hoppers of a multispindle automatic lathe (6 or 8 spindles) and a rolling bearing ring is machined from them. The rings are subsequently heat-treated, i.e. hardened and tempered. Since the rings are oxidized, with the formation of scale, and distorted during this heat treatment, the ring has to be ground to the final dimensions. The rolling bearing producer then carries out the assembly of a rolling bearing by joining together the outer and inner rings, rolling elements and cage or cover plates.
To reduce the production costs in the preliminary stages, it has already been proposed (DE 195 20 833 A1) to feed the continuously cast material to a tube producing installation in the cast state and without heat treatment (homogenization).
A further step in this direction was the elimination of long-time annealing (spheroidizing) by controlled final, rolling in a specific temperature range with predetermined degrees of deformation, which comes very close to TM rolling (DE 195 13 314 A1). As an alternative to the previously known cutting off of the rings in the course of machining cold-pilger-rolled or cold-drawn tubes, a so-called blank ring can be produced by hot-cutting (WO 95/29777). All the proposals made lead to a cost reduction in the production of rolling bearing rings.
The object of the present invention is to provide a method for producing rolling bearing rings which is based on this known prior art and makes a still further cost reduction possible.
The essence of the invention is the cost-effective use of a hot-fabricated tube as the starting material in conjunction with hot-cutting for producing a blank ring, from which a green or soft ring of a rolling bearing ring, i.e. an outer or inner ring, is produced cost-effectively by means of an efficient form of cold further processing. The main element of the cold further processing is a turning operation which allows high cutting speeds and consequently short cycle times. The cost-intensive production of a cold-pilger-rolled or cold-drawn tube as the starting product for ring production is no longer needed with the method according to the invention, and the same applies to all the associated transporting and handling steps.
Proposed as the optimum sequence is to feed the hot-fabricated tube from the cooling bed to the device for hot-cutting without any further subsequent treatment, in particular without straightening, without heat treatment and without non-destructive testing, and to produce a rolling bearing ring from the blank ring in just one single device with just one re-chucking step by means of an optimally timed multistage turning operation and to test this ring with regard to its dimensions and freedom from defects. To keep down the required amount of machining with a view to short cycle times ( less than 8 sec), the blank ring should come as close as possible to the desired ideal state in its dimensions and its contour. This is not always the case, however, for process-related reasons. For example, the two end faces are not always exactly plane-parallel. Furthermore, formation of burrs toward the inside in the drilling region usually cannot be entirely avoided, since the hot-cutting takes place without any internal support. In view of these circumstances, it has been found to be advantageous for the actual cold further processing to be preceded by a punching and pressing operation and/or partial finish-grinding. Partial finish-grinding in the case of an outer ring comprises grinding the two end faces and/or the circumferential surface of the blank ring to soft ring dimensions. In this way, the multistage turning can take place in an optimum range and tool wear is less. In particular, the preceding finish-grinding of the outer diameter of the blank ring makes it possible to dispense with turning of the outside diameter to the final dimensions and consequently to save a working step which constitutes an increase in the cycle time. Furthermore, in this case only one chucking step is required, so that there is no need for re-chucking. The additional effort of carrying out the preceding punching and pressing operation and/or the partial finish-grinding must be weighed up against the increase in efficiency of the turning operation. Alternatively, the pretreated blank ring may also be machined on customary lathes with chucking twice. A further useful means for increasing machining efficiency is to supply a lubricant. It has been found to be particularly advantageous for a mixture of compressed air and customary drilling emulsion to be supplied in a pulsed manner. In this connection, in a pulsed manner is intended to mean that the supply takes place only during the actual machining operation and remains switched off during cycle advancement and bringing the tools into position.
The otherwise customary heat treatment of the so-called green or soft ring can be advantageously integrated into the machining process. For example, this heat treatment may be carried out directly after the punching and pressing operation. Another possibility is to carry out the hot-cutting at a temperature above the transformation temperature and to quench the blank ring immediately thereafter, for example letting it drop into a water bath or oil bath. After the subsequent tempering, the blank ring has the hardness required for the rolling bearing. The machining-type cold further processing is then a hard turning operation and/or a hard grinding operation, for which corresponding process technology has recently been developed.
It can be regarded as the advantage of the proposed procedure that, while retaining the cost advantages arising from
the direct use of extruded material
controlled final rolling of the heat-fabricated and
hot-cutting,
an additional cost advantage is achieved by the cold further processing adapted to the preliminary stages. The advantages arising from the preliminary stages, such as higher degree of purity, lower surface decarburization, less distortion during the subsequent heat treatment (less finish grinding allowance) and longer service life on account of the fine-grained nature of the structure, which is retained with this method, can be used along with the new advantages, so that a corresponding overall advantage is obtained.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.